2 Answers
2

I know this isn't a satisfying answer, but I would say that the most likely reason is that this is a plot hole mistake by the filmmakers.

To tell the truth, I never realized this plot inconsistency before seeing your question, because I was always more bothered by the fact that they want to launch the ArcNet into space, but they attempt to do so by placing it on the escape rocket, which is part of the rocket that never goes into outer space. The escape rocket is jettisoned once the rocket reaches a safe altitude, so if they place the ArcNet on the escape rocket, they are guaranteeing that the ArcNet will end up at the bottom of the Atlantic.

So yes, they would have been far better off asking one of their alien friends to put it in space with their own spaceships.

Ans.- In altered timeline the shield does not exist. Remember what O replied in response to Js asking for the shield to stop the Boglodite invasion. She said:

No K. No defense system. So this option is out.

2) Why did not the aliens put the shield in the past(where J traveled back)?

There may be two reasons for that.

A) Boris changed the past resulting change in present where earth is, in the altered timeline, now vulnerable to the attack of the Boglodite, as the shield is not present there. Now in reality Agent J was the one who was responsible for covering the earth with the shield. So he must take the responsibility again to put the shield. Like Griff said:

The bitterest truth is better than the sweetest lies.

He has to go to Cape Canaveral.

He must deploy the Arc Net as he did before.

There's no other way.K is the only hope in saving Earth. Can I save him?

Is there any future where I save his life?

Yes.

But where there is death......there will always be death.

So you see there was always J who was responsible(and experienced too) for putting up the shield.

Now this is an obvious logic behind the act. But I want to add another theory on my own.

B) We know that Bogoldite has the ability to destroy each planet and their inhabitants. Now if they requested any friendly alien species to do the job, there are several harmful possibilities which arise with this.

i) Threatened or bribed by Boris, the species may hand over the Shield to him.

ii) It may think to put up the shield on its own planet in lieu of the earth to survive from Bogoldite because at the end of the day that planet is their home.

iii) The most important reason is Earth is home of humans and not the aliens. So there may be lack of enough try from them which might cause in disaster.